1. Field of the Invention
The present invention relates to a time-of-flight mass spectrometer with an ion trap bound thereto and, more particularly, to a mass spectrometer for proteome analysis.
2. Description of the Background
In the field of proteome analysis, the so-called “shotgun method” is in wide use, which comprises decomposing a protein mixture extracted from cells with a digestive enzyme, separating the fragment peptides obtained using a liquid chromatograph, selecting, within a mass spectrometer, one peptide species and decomposing this by collision-induced dissociation (CID), determining the molecular weights of the resulting fragments from a mass spectrum of the fragments, and identifying the original protein by checking against a genome database. The technique comprising selecting and decomposing one ion species within a mass spectrometer and subjecting the fragments to mass spectrometry is generally called “MS/MS analysis.” In some kinds of mass spectrometers, it is possible to select one fragment among the fragments resulting from MS/MS analysis and further subjecting that fragment to MS/MS. It is also possible to repeat such sequence n times, and this technique is generally called “MSn analysis.”
A quadrupole ion trap mass spectrometer (ITMS) can perform MSn analysis where n is not less than 3, and is characterized in that high levels of sensitivity and efficiency can be attained because CID is performed after accumulation of ions in the ion trap. In proteome analysis, however, mass-to-charge ratio ranges of up to about 3,000 and a mass resolution of at least about 5,000 are desired, whereas the conventional ion trap mass spectrometers are generally about 2,000 in mass-to-charge ratio and in mass resolution and have a decreased mass accuracy. Hence, the range of application of conventional ITMS is limited, and only low protein identification efficiency can be secured with such apparatuses.
In B. M. Chien, S. M. Michael and D. M. Lubman, Rapid Commun. Mass Spectrom. Vol.7 (1993) 837, there is disclosed a mass spectrometer comprising a quadrupole ion trap and a time-of-flight mass spectrometer (TOFMS) that are coaxially combined. When this apparatus is used, it is possible to perform MSn analysis (n being not less than 3) at high levels of mass-to-charge ratio ranges and mass accuracy using the TOFMS.
However, because, in this apparatus, the ion trap and the TOFMS are combined coaxially and the ion trap also serves as an accelerator for the TOFMS, a collision of ions with the neutral gas for CID occurs frequently during acceleration. The ions are thereby scattered and, as a result, it is difficult to attain a high level of resolution. However, when the acceleration voltage is increased, it becomes possible to eject ions in a shorter time and to thereby reduce the scattering thereof. Hence, the resolution may be improved, but there arises the problem that the collision energy increases and, as a result, ions are readily decomposed. When ions are decomposed during acceleration, chemical noises are produced, whereby the lower detection limit is deteriorated.
In the mass spectrometer described in U.S. Pat. No. 6,011,259, CID is effected in a multi-pole ion guide, and the resulting ions are discharged from the ion guide and analyzed in a TOFMS of the orthogonal accelerator type. Because the orthogonal accelerator can be disposed in a high vacuum region, the frequency of collisions with a neutral gas during acceleration is substantially negligible. Generally, the efficiency of CID in a multi-pole ion guide is lower as compared with ion traps. However, the CID efficiency can be improved to some extent by causing the ion guide to function as a two-dimensional ion trap (also called a linear trap).
However, the space distribution and energy distribution of ions relative to the axial direction of the ion guide are large, and, therefore, the ions accelerated are dispersed. As a result, there arises the problem that the detection sensitivity is low. Unlike the quadrupole ion trap, the linear trap cannot be used in MSn where n is not less than 3.
In C. Marinach, A. Brunot, C. Beaugrand, G. Bolbach, J. C. Tabet, Proceedings of the 49th ASMS Conference on Mass Spectrometry and Allied Topics, Chicago, Ill., May 27-31, 2001, there is disclosed a mass spectrometer in which a quadrupole ion trap and a TOFMS are combined off axis. In this apparatus, ions are initially ejected from the ion trap, then accelerated in a direction perpendicular to the axis of the ion trap, and finally subjected to analysis on the TOFMS. In this apparatus, ions spatially focused in the middle of the ion trap are dispersed as far as possible relative to the axial direction during transfer thereof from the ion trap to the orthogonal accelerator. This causes the ions to form a continuous ion flow while an acceleration voltage pulse is continuously applied at spaced intervals (i.e., repeated pulses) to perform analysis on the TOFMS. Since ions spatially and energetically focused within the ion trap are converted to a continuous ion flow, there arises, as a result, the same problems as with the apparatus described above with reference to U.S. Pat. No. 6,011,259.
As discussed above, the prior art mass spectrometers are characterized in that it is difficult to simultaneously attain broad mass-to-charge ratio ranges and high mass resolution with sufficient detection sensitivity.